<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <script src="vue.js"></script>
    <title>Title</title>
</head>
<body>
</body>
</html>
<script>
    var data = { a: 1 };
    var app = new Vue({
        el: '#app',
        data: data,
        created: function () {
            console.log('a is: ' + this.a)
        }
    });
    console.log(app.a === data.a);//true
    app.a = 2;
    console.log(data.a);//2
    data.a = 3;
    console.log(app.a);//3
    console.log(app.$data === data);//true
    console.log(app.$el === document.getElementById('app'));//true
</script>